Method of making fibre-based products and their use

ABSTRACT

Thermoplastic fibres of different melting points are present in an aqueous slurry of refractory fibres, binder and flocculant. The slurry is passed through a screen to form a wet body which is heated to dry the body and melt the thermoplastic fibres to bind the refractory fibres together.

[0001] The invention relates to a method of making fibre-based productsand their use. One particular application of the invention is themanufacture of a mat like article to hold a catalytic converter inside acan at high temperature.

[0002] According to one aspect of the invention there is provided amethod of making a fibre-based article, the method comprising:

[0003] forming an aqueous slurry containing a major proportion ofrefractory fibres and a minor proportion of thermoplastic fibres whichcomprises fibres having two different melting points, one lower than theother;

[0004] adding a binder and a flocculating agent;

[0005] passing the slurry containing the refractory fibres,thermoplastic fibres, binder and flocculating agent through a perforatescreen to deposit a body of wet fibres on the screen;

[0006] heating the body to remove the water and to melt the lowermelting point thermoplastic fibres to bind the refractory fibres to forma preform; and

[0007] thermomoulding the preform then cooling under pressure to formthe article.

[0008] In another aspect there is provided a dry preform articlecomprising a major proportion of refractory fibres having a mean lengthof about 4 cm and a mean diameter of up to about 6 micron, held togetherby melted thermoplastic fibres and having a substantially uniformdensity over its area.

[0009] Preferably the fibres making up the major proportion arerelatively short, say up to 4 cm in length and up to say 6 micron indiameter. The fibres are preferably inorganic fibres such as alumina,mullite, silica, zirconia; graphite: they may be used alone or withother materials such as intumenescent plate materials; particulates; orany materials like these. Combinations may be used. Most preferably thefibres are the ones available under the registered trade mark SAFFIL.

[0010] The thermoplastic fibres may be selected from a wide variety ofsuitable materials having different melting points. The choice isdetermined by compatability with the other fibres and the temperature atwhich the fibres melt. The function of this lower melting component isto bond the refractory fibres and hold them together until they areexposed to the temperature at which the thermoplastic fibres aredecomposed. Typically suitable fibres are olefins such as polyethyleneand polypropylene; polyesters: polyamines. It is preferred to usebicomponent fibres which have having a core and a sheath made ofdifferent plastics. Suitable materials are available from differentsuppliers, e.g. “Trevira 255” available from Trevira Fasern, HartmannHuth, Frankfurt, Germany and “Adhesion C special” fibres available fromFibrevisions. Denmark. The concentration of thermoplastic fibres in thefinal product may vary; generally we prefer about 10% by weight.

[0011] The binder is preferably a latex; the flocculating agent ispreferably a polyelectrolyte.

[0012] When the deposited body of wet fibres is heated the water isremoved and the lower melting point thermoplastic material melts to bindthe fibres to form a substantially self-supporting body. The refractoryfibres are anchored in the solidified thermoplastic. Because of the wayin which the article has been made it has substantially the sameconsistency and density throughout. The thermoplastic gives the matintegrity making it easier to handle and process.

[0013] A further step of thermoulding under compression provides a denseinorganic fibre mat of bulk density suitable for use, e.g. in themonolith in the application.

[0014] The process may be batch or continuous.

[0015] The article may be used as a support between the inner surface ofa catalytic convertor can and the convertor itself. The article may bein the form of mat up to say 8 mm thick which can be used in number ofknown “canning” techniques. When the convertor is first heated thethermoplastic will burn away to provide a porous resilient structurewhich holds and supports the catalytic body in place over a prolongedperiod despite the vibration and fluctuations in temperature. Theresulting composition of fibres or fibres (and intrumescent particulatesif present) exerts a pressure which supports the delicate convertorduring its life. An advantage of the invention is that because it is soself-supporting and resistant to high temperature it is possible to usea mat of the invention in a high temperature environment, for example ina close coupled application, e.g. near an engine exhaust manifold. Themat may be used in vehicles having gasoline, Diesel engines or the like.

[0016] In order that the invention may be well understood it will now bedescribed by way of illustration only with reference to the followingexamples.

EXAMPLE 1

[0017] Alumina fibres measuring about 3 micron in diameter and having amean length of about 4 cm were added to water in a concentration ofabout 0.1% by weight to form a slurry. The solids were prevented fromsettling by gentle agitation. Bicomponent fibres having a core ofpolypropylene and a sheath of polyethylene and of a size compatible withthe alumina fibres were added in a concentration of 0.01% by weight. Astandard acrylate latex was added and dispersed, followed by aflocculating agent. The mix was added to a vessel having a perforatefloor, and the water was allowed to flow through leaving a pad-likedeposit on the floor. This was allowed to dry and removed to an oven andheated to about 130° C. to fully dry the deposit and to melt thethermoplastic sheath of the bicomponent fibres to bond the aluminafibres together. The dry mat was then heated to 150° C. and cooled undercompression to a thickness of about 5 mm. The formed mat was found to beof substantially uniform density across its area.

[0018] Portions of the mat were cut into shapes for an infill for amonolith support. The shapes were mounted in a can which was positionedin the style of a close coupled catalyst. When the catalyst was heatedby the exhaust gases the thermoplastic reached its decompositiontemperature and burned out, giving the mat a resilient porous fibrestructure. The support mat was well able to hold the catalyst in thecan, despite vibration and variation in temperature.

EXAMPLE 2

[0019] Alumina fibres measuring about 3 micron in diameter and having amean length of about 4 cm were added to water in concentration of about0.1% by weight to form a slurry. The solids were prevented from settlingby gentle agitation. Bicomponent fibres having a core of polyethyleneterphthalate and a sheath of polyethylene and of a size compatible withalumina fibres were added in a concentration of 0.01% by weight. Astandard acrylate latex was added and dispersed followed by aflocculating polyelectrolyte agent. The mix was added to a vessel havinga perforate floor and the water was allowed to flow through leaving apad-like deposit on the floor. This was allowed to dry and removed to anoven and heated to about 130° C. to fully dry the deposit and to meltthe thermoplastic sheath of the fibres to bond the alumina fibrestogether. The dry mat was then heated to 150° C. and cooled undercompression to a thickness of about 5 mm. The formed mat was found to beof substantially uniform density across its area.

[0020] Portions of the mat were cut into shapes for an infill for amonolith support. The shapes were mounted in a can which was positionedin the style of a close coupled catalyst. When the catalyst was heatedby the exhaust gases, the thermoplastic reached its decompositiontemperature and burned out giving the mat a resilient porous fibrestructure. The support mat was well able to hold the catalyst in thecan, despite vibration and variation in temperature.

1. A method of making a mat having a substantially consistent densityacross its area, the method comprising: forming an aqueous slurrycontaining a major proportion of refractory fibres and a minorproportion of thermoplastic fibres which comprise fibres having twodifferent melting points, one lower than the other; adding a binder anda flocculating agent; passing the slurry containing the refractoryfibres, thermoplastic fibres, binder and flocculating agent through aperforate screen to deposit a body of wet fibres on the screen; heatingthe body to remove the water and melt the lower melting point fibres toform a preform; and thermoforming the preform under pressure to form thearticle.
 2. A method according to claim 1, wherein the thermoplasticfibres comprise bicomponent fibres comprising a core and a sheath.
 3. Amethod according to claim 2, wherein the core is polypropylene orpolyethylene terephthalate and the sheath is polyethylene.
 4. A methodaccording to any preceding claim, wherein the refractory fibres range upto 4 cm in length and up to 6 micron in diameter.
 5. A method accordingto any preceding claim, wherein the binder is a latex.
 6. A methodaccording to any preceding claim, wherein the flocculating agent is apolyelectrolyte.
 7. A method according to any preceding claim, whereinthe refractory fibres are present in a concentration of about 0.1% byweight in the slurry.
 8. A method according to any preceding claim,wherein the thermoplastic fibres are present in a concentration of about0.01% by weight in the slurry.
 9. A method according to any precedingclaim, wherein the preform is heated to 150° C. and cooled undercompression.
 10. A dry preform article comprising a major proportion ofrefractory fibres having a mean length of about 4 cm and a mean diameterof about 6 micron, held together by melted thermoplastic fibres andhaving a substantially uniform density over its area.